1. Technical Field of the Invention
The present invention relates to optical fiber amplifiers which are suitable for use as optical amplifiers in the fields of optical communications, optical measurement, and the like.
2. Background Art
Semiconductor laser amplifiers and rare-earth-doped optical fibers (hereinafter referred to as EDFs) and the like are examples of optical amplifiers which amplify and output inputted optical signals while in the form of light. However, such optical amplifiers have the drawback that their gain depends upon the power of the input optical signal. Specifically with regard to EDF amplifiers, the gain decreases as the power of the input optical signal increases. Various gain control methods have been proposed for overcoming this drawback.
In one example of such a gain control method, the input optical signal and the output optical signal are simultaneously measured in order to make a comparison between the input beam and the output beam. Based on these comparison signals, feedback is applied to the drive current of the laser diode in the case of semiconductor laser amplifiers and to the electrical power of the excitation beam source (the light source required for amplification) in the case of EDF amplifiers, in order to control the gain so as to hold it constant.
As another example in the case of EDF amplifiers, the power of the excitation beam entering the EDF and the power of the beam emitted by the excitation beam source of the EDF are simultaneously measured. Their ratio is calculated to obtain the gain of the EDF amplifier, which is then used to apply feedback to the power of the excitation beam source in order to control the gain so as to hold it constant.
Thus, in conventional EDF amplifiers, the gain is controlled by adjusting the power of the excitation beam based on calculated gain values in order to apply feedback to the EDF. However, there is a problem in that the gain is not able to be controlled at high speeds because the response rate of the population inversion of the excitation beam in the EDF is too slow. More specifically, in an EDF, the relaxation time of spontaneous emission from the upper state .sup.4 I.sub.13/2 to the lower state .sup.4 I.sub.15/2 (approximately equal to the lifetime of the upper state .sup.4 I.sub.13/2) is about 10 milliseconds. Consequently, even if the power of the excitation beam source is changed, the population inversion of the EDF is not able to catch up fast enough.
Furthermore, with the method wherein the gain is controlled by comparing the powers of the input and output optical signals, the gain cannot be controlled at a constant value when there is no input optical signal, so that when the next optical signal is inputted, the gain is not able to be controlled very quickly.